Degranulation Of Dermal Mast Cells Research Articles

To Evaluate the Role and Relevance of Cytokines IL-17, IL-18, IL-23 and TNF-α and Their Correlation with Disease Severity in Chronic Urticaria.

Introduction:The basic event in the pathogenesis of urticaria is inappropriate activation and degranulation of dermal mast cells. Cytokines are soluble polypeptide mediators that play a key role in immunological, inflammatory and reparative host responses including chronic urticaria.Objective:The aim of this study was to evaluate the role and relevance of cytokines interleukin-17 (IL-17), interleukin-18(IL-18), interleukin-23(IL-23) and tumor necrosis factor-alpha (TNF-α) and their correlation with disease severity in patients with chronic urticaria.Materials and Methods:A prospective cross-sectional study was conducted to measure the serum concentration of IL-17, IL-18, IL-23 and TNF-α in 50 chronic urticaria patients and in 30 healthy controls. Disease activity was assessed by using urticaria activity score (UAS).Results:Serum concentration of IL-17, IL-18, IL-23 and TNF-α were significantly higher during the acute episode in chronic urticaria patients as compared with the healthy control subjects (mean: 1.84 ± 0.81 vs 0.03 ± 0.02 pg/ml; P < 0.001, 501.41 ± 208.98 vs 218.39 ± 39.83 pg/ml; P < 0.001; 25.57 ± 10.79 vs 0.15 ± 0.14 pg/ml, P < 0.001; and 455.54 ± 253.54 vs 8.498 ± 3.644 pg/ml, P < 0.001, respectively). There was a significant positive correlation between serum levels of IL-17, IL-18, IL-23 and TNF-α and severity of disease.Conclusion:The serum levels of IL-17, IL-18, IL-23 and TNF-α were raised in patients of chronic urticaria and positively correlated with the severity of urticaria.

Dermoscopy of Urticaria Pigmentosa.

Dermoscopy of Urticaria Pigmentosa.

Radiation-induced mast cell mediators differentially modulate chemokine release from dermal fibroblasts

Ionizing radiation has been demonstrated to result in degranulation of dermal mast cells. Chemokines are thought to play a crucial role in the early phase of the cutaneous radiation reaction. In human skin, mast cells are located in close proximity to dermal fibroblasts, which thus are a potential target for the action of mast cell mediators. In this study, we evaluated the effects of mast cell-derived histamine, serotonin, tumour necrosis factor (TNF)-α and tryptase on chemokine release from dermal fibroblasts. Human mast cells (HMC-1) were investigated for histamine release and cytokine production after ionizing radiation using enzyme-linked immunosorbent assay (ELISA) and flow cytometry. Receptor expression on human fetal foreskin fibroblasts (HFFF2) and human adult skin fibroblasts (HDFa) was examined by flow cytometry. Chemokine mRNA and protein expression were analyzed by gene array and ELISA, respectively. Ionizing radiation significantly increased histamine release and cytokine expression by HMC-1 cells. Receptors for histamine, serotonin, TNF-α and tryptase were detected both in HFFF2 and in HDFa cells. Dermal fibroblasts constitutively expressed distinct sets of chemokine mRNA. Mast cell mediators differentially affected the release of chemokines CCL8, CCL13, CXCL4 and CXCL6 by fibroblasts. Our data suggest that radiation-induced mast cell mediators have a tremendous impact on inflammatory cell recruitment into irradiated skin. We postulate the activation of mast cells to be an initial key event in the cutaneous radiation reaction, which might offer promising targets for treatment of both normal tissue side effects in radiation therapy and radiation injuries.

ONE MECHANISM BEHIND LED PHOTOTHERAPY FOR WOUND HEALING AND SKIN REJUVENATION: KEY ROLE OF THE MAST CELL

Light-emitting diodes (LEDs) have attracted a lot of attention in light-only skin rejuvenation and wound healing with an 830 nm/633 nm combination, but the mechanisms by which LED therapy speeds up the healing process and increases collagen synthesis remain unclear. One hypothesis, examined in the present study, concerns the interaction between 830 nm near infrared light and the degranulation of dermal mast cells. The left forearm of 8 healthy male subjects was irradiated with an 830 nm LED array (20 min, 57 J/cm 2 ) with the right as the unirradiated control. Biopsies were taken before and two days after irradiation and routinely prepared for transmission electron microscopy (TEM), and compared between baseline, irradiated and unirradiated tissue. The TEM in all postirradiated specimens, while clearly showing no damage to the irradiated tissue with all tissue components essentially morphologically normal, demonstrated a mild inflammatory response 48 hr after 830 nm irradiation with interstitial and perivascular oedema. A number of macrophages and leukocytes had been recruited into the irradiated tissue, and mast cells had increased in number and had either degranulated or were in the process of doing so. The unirradiated control tissue showed no such changes. The TEM findings in the present study showed a clearly-visible inflammatory response similar to the first phase of wound healing, a ‘quasi-wound’, but created athermally and atraumatically following a single treatment with 830 nm light, thereby kick-starting the inflammatory stage of the wound healing process which is recognized as absolutely necessary in achieving good subsequent collagen synthesis in the second phase of proliferation, followed by good remodeling in the third phase. Good results in skin rejuvenation, both ablative and non-ablative, have been well-linked to establishing the wound healing process. The 830 nm-mediated ‘quasi-wound’ may well be an essential element in light-only LED photorejuvenation. Furthermore, the action of 830 nm on the inflammatory cells in actual wounds will significantly accelerate the wound healing process, controlling and peaking inflammation, and allowing proliferation to occur sooner and more efficiently.

Ionizing radiation induces degranulation of human mast cells and release of tryptase

Purpose: Skin fibrosis is a hallmark of ionizing radiation-induced tissue injury and we hypothesized that mast cells via their products (especially tryptase) are involved in this event. We therefore investigated whether: (i) irradiation with 5 Gray (Gy) is able to induce the release of the typical mast cell mediator tryptase from human mast cells (HMC-1) in vitro, (ii) this effect can be influenced by application of clinically relevant mast cell blockers, and (iii) irradiation leads to mast cell degranulation in ex vivo skin culture models.Materials and methods: The human mast cell line (HMC)-1, as well as ex vivo skin tissue served as experimental models. Fluorescence activated cell sorting (FACS), Enzyme linked immunosorbent assays (ELISA), mast cell degranulation assays and immunohistochemistry were applied.Results: Ionizing radiation induces a time-dependent, statistically significant increase in the release of tryptase by HMC-1 cultured in vitro. Mast cell degranulation and secretion of tryptase was partially, but not significantly, inhibited by pre-incubation with the histamine-1 receptor (H1) blocker cetirizine. Mast cell degranulation was also clearly evident after irradiation using an ex vivo skin culture model of mastocytoma tissue.Conclusions: We propose that ionizing radiation leads to a degranulation of dermal mast cells, an event which is accompanied by the release of tryptase.

Role of substance P in stress-derived degranulation of dermal mast cells in mice

The interaction between nerves and mast cells can effect regulation of the immune system and inflammatory responses. Recent studies have shown that various stressors can induce degranulation of dermal mast cells in animals. This study was conducted to confirm that substance P (SP) was involved in the degranulation of dermal mast cells in stress conditions. Using a communication box system, foot shock stress (FS) and psychological stress (PS) were administered to mice and the degranulation rate of dermal mast cells, the number of SP-positive nerve fibers and changes in SP content were determined. The inhibitory effect of a non-peptide NK1-receptor antagonist on these changes was investigated. Both FS and PS significantly enhanced the degranulation of dermal mast cells and increased the number of SP-positive nerve fibers. FS significantly decreased dermal SP content whereas SP was increased by PS. These changes were inhibited by intraperitoneal injection of NK(1) receptor antagonist. It was considered that SP released from the nerve ending, had an important role in the degranulation of dermal mast cells. Results of this study suggest that the tachykinin receptor antagonist exhibited an inhibitory effect on aggravated stress-induced dermatitis.

Age-Induced Reprogramming of Mast Cell Degranulation

Mast cell degranulation can initiate an acute inflammatory response and contribute to the progression of chronic diseases. Alteration in the cellular programs that determine the requirement for mast cell degranulation would therefore have the potential to dramatically impact disease severity. Mast cells are exposed to increased levels of PGE2 during inflammation. We show that although PGE2 does not trigger the degranulation of dermal mast cells of young animals, in older mice, PGE2 is a potent mast cell stimulator. Intradermal administration of PGE2 leads to an EP3 receptor-dependent degranulation of mast cells, with the number of degranulated cells approaching levels observed in IgE- and Ag-treated controls. Taken together, these studies suggest that the ability of PGE2 to initiate mast cell degranulation changes in the aging animal. Therefore, elevated PGE2 levels might provide an important pathway by which mast cells are engaged to participate in inflammatory responses in the elderly patient.

Angioedema

Although first described more than 130 years ago, the pathophysiology, origin, and management of the several types of angioedema are poorly understood by most dermatologists. Although clinically similar, angioedema can be caused by either mast cell degranulation or activation of kinin formation. In the former category, allergic and nonsteroidal anti-inflammatory drug-induced angioedema are frequently accompanied by urticaria. Idiopathic chronic angioedema is also usually accompanied by urticaria, but can occur without hives. In either case, an autoimmune process leading to dermal mast cell degranulation occurs in some patients. In these patients, histamine-releasing IgG anti-FcepsilonR1 autoantibodies are believed to be the cause of the disease, removal or suppression by immunomodulation being followed by remission. Angiotensin-converting enzyme inhibitor-induced angioedema is unaccompanied by hives, and is caused by the inhibition of enzymatic degradation of tissue bradykinin. Hereditary angioedema, caused by unchecked tissue bradykinin formation, is recognized biochemically by a low plasma C'4 and low quantitative or functional C'1 inhibitor. Progress has now been made in understanding the molecular genetic basis of the two isoforms of this dominantly inherited disease. Recently, a third type of hereditary angioedema has been defined by several groups. Occurring exclusively in women, it is not associated with detectable abnormalities of the complement system. Angioedema caused by a C'1 esterase inhibitor deficiency can also be acquired in several clinical settings, including lymphoma and autoimmune connective tissue disease. It can also occur as a consequence of specific anti-C'1 esterase autoantibodies in some patients. We have reviewed the clinical features, diagnosis, and management of these different subtypes of angioedema.After completing this learning activity, participants should be aware of the classification, causes, and differential diagnosis of angioedema, the molecular basis of hereditary and non-hereditary forms of angioedema, and be able to formulate a pathophysiology-based treatment strategy for each of the subtypes of angioedema.

Dermal mast cells play a central role in the incidence of scratching behavior in mice induced by multiple application of the hapten, 2,4,6‐trinitrochlorobenzene

Repeated application of 1% 2,4,6-trinitrochlorobenzene (TNCB) in acetone solution causes chronic skin inflammation in BALB/c mice. Associated scratching behavior gradually appeared, and chronic scratching behavior was established over 40 days after the initial application of TNCB. In order to explore the possible involvement of T cells and mast cells in the appearance of pruritus, we examined the response of athymic nude mice and genetically mast-cell-deficient mice. We could not detect either severe skin inflammation or immunoglobulin (Ig)E production in T-cell-deficient BALB/c nu/nu mice even after 80 days of TNCB treatment, whereas typical severe skin inflammation and IgE production were observed in mast-cell-deficient WBB6F1-W/Wv and WBB6F1-Sl/Sld mice. Furthermore, we observed persistent scratching behavior in WBB6F1-W/Wv mice, but not in BALB/c nu/nu and WBB6F1-Sl/Sld mice. Histological examination of TNCB-treated animals revealed the development of dermal mast cells in W/Wv mice but not in Sl/Sld mice. Degranulation of dermal mast cells was observed in the WBB6F1-W/Wv genotype, but most mast cells remained intact in TNCB-treated BALB/c nu/nu mice. These results suggest that mast cells play a pivotal role in the incidence of scratching behavior in this chronic pruritus model.

Anopheles Mosquito Bites Activate Cutaneous Mast Cells Leading to a Local Inflammatory Response and Lymph Node Hyperplasia

When Anopheles mosquitoes probe the skin for blood feeding, they inject saliva in dermal tissue. Mosquito saliva is known to exert various biological activities, but its perception by the immune system and its role in parasite transmission remain poorly understood. In the present study, we report on the cellular changes occurring in the mouse skin and draining lymph nodes after a Anopheles stephensi mosquito bite. We show that mosquito bites induce dermal mast cell degranulation, leading to fluid extravasation and neutrophil influx. This inflammatory response does not occur in mast cell-deficient W/W(v) mice, unless these are reconstituted specifically with mast cells. Mast cell activation caused by A. stephensi mosquito bites is followed by hyperplasia of the draining lymph node due to the accumulation of CD3(+), B220(+), CD11b(+), and CD11c(+) leukocytes. The T cell enrichment of the draining lymph nodes results from their sequestration from the circulation rather than local proliferation. These data demonstrate that mosquito bites and very likely saliva rapidly trigger the immune system, emphasizing the critical contribution of peripheral mast cells in inducing T cell and dendritic cell recruitment within draining lymph nodes, a prerequisite for the elicitation of T and B lymphocyte priming.

Classification of anti-FcepsilonRI and anti-IgE autoantibodies in chronic idiopathic urticaria and correlation with disease severity.

Circulating autoantibodies against FcepsilonRI, IgE, or both occur in approximately one third of patients with chronic idiopathic urticaria (CIU), but not all autoantibodies initiate histamine release. We sought to classify patients with CIU into subsets on the basis of serum bioactivity and immunoreactivity and to examine the relationship between newly defined subtype and disease severity. Sera from patients with CIU (n = 78), dermog-raphism (n = 15), and cholinergic urticaria (n = 10) and sera from healthy subjects (n = 39) were analyzed by means of Western blot analysis for anti-FcepsilonRI autoantibodies and for histamine release from basophils and dermal mast cells. In vivo reactivity of autologous serum was tested by means of intradermal injection, and CIU severity was determined on the basis of clinical interview. We classified sera from patients with CIU into 5 subsets: immunoreactive histamine-releasing anti-FcepsilonRI autoantibodies (n = 20 [26%]); immunoreactive anti-FcepsilonRI autoantibodies without histamine-releasing activity (n = 12 [15%]); anti-IgE-like autoantibodies (n = 7 [9%]); serum containing a mast cell-specific histamine-releasing factor (n = 7 [9%]); and sera with no identifiable factor (n = 32 [41%]). Patients with serum histamine-releasing activity had more severe urticaria than patients without such activity. Positive skin test responses to autologous sera were associated with histamine-releasing anti-FcepsilonRI autoantibodies but not with non-histamine-releasing anti-FcepsilonRI autoantibodies. Neither healthy subjects nor patients with dermographism or cholinergic urticaria had his-tamine-releasing anti-FcepsilonRI autoantibodies. These data support the specificity of functional anti-FcepsilonRI autoantibodies to CIU. The identification of distinctive subsets of patients suggests that other pathogenic mechanisms occur in CIU in addition to direct ligation of FcepsilonRI by autoantibodies causing dermal mast cell degranulation. Elucidating these mechanisms might lead to new treatments for CIU.

Ultrastructural analysis of human skin biopsy specimens from patients receiving recombinant human stem cell factor: Subcutaneous injection of rhSCF induces dermal mast cell degranulation and granulocyte recruitment at the injection site

We performed an ultrastructural analysis of 10 skin biopsy specimens that had been obtained from three women who were undergoing daily subcutaneous dosing with recombinant methionyl-human stem cell factor (rhSCF) as part of a phase I clinical trial. The biopsy specimens were obtained at sites of subcutaneous administration of rhSCF, within approximately 1 to 2 hours of rhSCF injection, and, at the same time, at contralateral control sites that had not been directly injected with rhSCF. We previously reported that subcutaneous dosing with rhSCF in these subjects induced the local development of a wheal and flare response, which was associated with evidence of mast cell degranulation, as well as a systemic increase in numbers of cutaneous mast cells. The present electron microscopic analysis revealed that all biopsies of swollen, erythematous rhSCF-injected sites exhibited anaphylactic degranulation of both mature and immature mast cells, an acute inflammatory response characterized by the migration of neutrophils, basophils (some of which exhibited evidence of piecemeal degranulation), and eosinophils through blood vessel walls into the perivascular and extravascular spaces, and edema and fibrin deposition within the interstitium. By contrast, the control biopsies contained no evidence of mast cell degranulation or acute inflammation. However, both control and rhSCF-injected sites exhibited mast cells that were undergoing granule building and maturation. Thus at the doses tested in these subjects, subcutaneous injection of rhSCF induced anaphylactic-type degranulation of dermal mast cells at the injection site, with an acute inflammatory response that was associated with the recruitment of granulocytes. By contrast, mast cells at sites distant from those directly injected with rhSCF exhibited no evidence of enhanced secretion. (J Allergy Clin Immunol 1998;101:793-806.)

Mast cells augment lesion size and persistence during experimental Leishmania major infection in the mouse.

Mast cells are a source of a variety of cytokines that may influence the host response to Leishmania major. To investigate the role of mast cells during L. major infection, we performed a morphometric analysis of mast cells at cutaneous sites in resistant C57BL/6 mice and susceptible BALB/c mice injected with L. major. Extensive dermal mast cell degranulation was found at sites of L. major infection in both strains of mice. We also examined the course of L. major infection in genetically mast cell-deficient WBB6F1-W/Wv or WCB6F1-Sl/Sld mice, their respective congenic normal (WBB6F1-(+/+) or WCB6F1-(+/+)) littermates, and WBB6F1-W/Wv mice that had been selectively and locally repaired of their cutaneous mast cell deficiency. We found that mast cells significantly augmented the intensity and maximal size of the cutaneous lesions at sites of L. major infection, and in some cases substantially prolonged the persistence of the reactions. However, the lesions ultimately resolved in both the mast cell-deficient and the congenic normal mice. In addition, the presence or absence of mast cells had little or no effect on the numbers of viable parasites recovered from the cutaneous lesions. Moreover, mast cell-deficient W/Wv mice and the congenic normal (+/+) mice produced similar levels of IFN-gamma mRNA in lymph nodes draining the cutaneous lesions whereas no IL-4 mRNA was detectable. Taken together, these data suggest that mast cells significantly augment the size of cutaneous lesions during L. major infection in mice. However, mast cells do not appear to influence significantly either the parasite burden or the ultimate resolution of the infection.

Dermal mast cell degranulation in systemic sclerosis

Paired biopsy samples from involved and uninvolved skin were obtained from 19 patients with generalized scleroderma (11 with early, progressive disease and 8 with late, improving disease). Skin biopsy samples were double stained for mast cell granules and for mast cell membrane. The number of mast cells was increased in patients with systemic sclerosis (SSc), in both involved and uninvolved skin and in both early and late disease. There was an increase in the number of degranulated mast cells in the involved skin of patients with both early and late disease and in the not-yet-involved skin of patients with early disease; however, there was no increase in the number of degranulated mast cells in areas of previously involved but now normal skin of patients with late disease. Increases in mast cell number and degranulation precede clinically apparent dermal fibrosis in SSc. These observations and the absence of mast cell degranulation in regressing skin suggest a participatory role of the mast cell in the clinical progression of skin changes in SSc.

Inflammatory properties of human C5a and C5a des Arg/ in mast cell-depleted human skin

C5a and its degradation product, C5a des Arg, elicit immediate cutaneous inflammatory reactions after intradermal injection. Histologically, these reactions are characterized by neutrophil-rich leukocytic infiltrates, leukocytoclasis, edema, and dermal mast cell degranulation. It has not been possible to assess in vivo the relative contributions of resident mast cells and circulating leukocytes to this reaction because the accumulation of leukocytes and degranulation of mast cells occur simultaneously after injection of these anaphylatoxins. To assess the role of mast cells in these inflammatory reactions, we have examined the reactivity of human skin selectively depleted of dermal mast cells by local corticosteroid treatment. Corticosteroid-treated skin became virtually devoid of dermal mast cells within 4–6 wk as assessed by light microscopy, immunofluorescence with fluorescein conjugated avidin, or electron microscopy. Mast cell-depleted skin demonstrated normal vasopermeability and vasodilatory responsiveness to intradermal injection of histamine, but the reactivity of these sites to the mast cell secretagogue, morphine, was absent. Moreover, no clinical reactions were detectable in mast cell-depleted human skin after intradermal challenge with 50 ng of either C5a or C5a des Arg, despite the fact that biopsies of these sites revealed substantial, neutrophil-rich infiltrates. These infiltrates were qualitatively and quantitatively identical to C5a or C5a des Arg-induced infiltrates in mast cell replete skin. This experimental approach in vivo has allowed the independent analysis of the anaphylactogenic and chemoattractant activities of human C5a and C5a des Arg in human skin, demonstrated the importance of dermal mast cells in these clinical responses, and shown that leukocytes can accumulate at these infection sites directly in response to these mediators.

A direct in vivo comparison of the inflammatory properties of human C5a and C5a des Arg in human skin.

C5a is an 11,000-Da complement-derived inflammatory glycoprotein that has been shown to mediate inflammatory reactions in vitro as well as in vivo in human skin. The C5a degradation product, C5a des Arg, is rapidly formed after exposure of C5a to serum carboxypeptidase N and may represent the relevant C5-derived inflammatory peptide in vivo. To examine the biologic activity of human C5a des Arg in vivo and to compare it with that seen with human C5a, we purified and characterized homogeneous preparations of human C5a and C5a des Arg and injected them intradermally into seven normal volunteers. C5a des Arg exhibited biochemical and biologic properties in vitro that were different from those of C5a. When injected into human skin, C5a des Arg was less potent than C5a, in respect to both minimal dose eliciting wheal and flare reactions and maximal wheal and flare elicited at a given dose, but C5a des Arg still elicited cutaneous wheal and flare reactions at physiologically relevant concentrations. Histologically, C5a des Arg skin test sites showed dense polymorphonuclear neutrophil-rich infiltrates associated with leukocytoclasis, dermal mast cell degranulation, and endothelial cell swelling. These were virtually indistinguishable from reactions elicited by C5a and occurred with concentrations attainable in vivo. Cutaneous wheal and flare reactions elicited by either C5a or C5a des Arg were partially inhibited by H1 antihistamines but were unaffected by selected nonsteroidal anti-inflammatory agents.

Induction of Mast Cell Degranulation in Skin by Ultrasound

Abstracf-Mast cells are numerous in the dermis and it is virtually inevitable that they will be exposed to ultrasound whenever it is used clinically. Their degranulation, in response to changes at their plasma membranes, induces inflammation. The degranulation of dermal mast cells in the intact skin of adult male Wistar rats (treated in vivo with ultrasound at intensities and frequencies within the therapeutic range) was examined quantitatively by light microscopy in formaldehyde-fixed sections stained with toluidine blue. The level of degranulation after treatment with ultrasound at spatial-average temporal-average intensities (I(SATA)) of less than 1.5 Wlcm* was similar to that of mockinsonated skin, while intensities (SATA) of 1.5 Wicm’ and over stimulated degranulation significantly. The effects of variation in I(SATA) and I(SAPA), where PA = pulse average intensity, and frequency sere examined. The results obtained indicate that the mechanism involved in the stimulation of mast cell degranulation by ultrasound at the levels investigated has a thermal component.

Histaminergic mediation of the response of rat skin to topical fluorides.

The topical application of sodium fluoride to abraded rat skin produced several morphological and biochemical effects. Related to the degranulation of dermal mast cells, skin histamine concentration was increased, fluorides were absorbed into the skin, and deposited mainly kin mitochondria. Dermal histamine binding was decreased for both H1 and H2 receptors with reduced binding sites, but epidermal adenyl cyclase was activated by fluorides. The response of the rat skin to fluorides involves a sequence of changes by which the potentiation of an inflammatory response also involves alterations in specific histamine receptors and a histamine-specific adenyl cyclase system.

Degranulation of dermal mast cells: effects of fixation and of antidromic nervous impulses on two histochemically identified cell-types.

Two types of mast cell can be identified histochemically in the dermis of the rat's external ear. One type is recognized by the binding of concanavalin A (con A) to the cytoplasmic granules (con A-positive cells) while in the other type (con A-negative cells), the granules do not bind con A. The granules in both types are stained metachromatically by toluidine blue. Antidromic stimulation of the great auricular nerve for 2 min results in an increased proportion of degranulating mast cells in the auricular dermis and both types of cell are affected to an approximately equal extent. In discussion of this observation, it is argued that both the con A-positive and the con A-negative mast cells are probably involved in the mediation of vasodilatation due to axon reflexes in injured skin. The proportions of degranulating mast cells determined in histological preparations varied with the fixatives (Carnoy and glutaraldehyde-formaldehyde) used, but the increased degranulation due to antidromic nervous stimulation could be detected after either fixation.

Action of adenosine triphosphate on mast cells in normal and denervated skin

Treatmentin vitro with adenosine triphosphate (ATP) causes degranulation of dermal mast cells in the skin of the rat's external ear. The magnitude of this effect increases with the concentration of ATP applied. In normal skin there is a marked increase in the proportion of degranulated mast cells with 2.5·10−6M ATP, while in skin denervated for 14 days this increase occurs with 1.25·10−6 M ATP. Thus, denervation is followed by hypersensitivity of the mast cells to ATP. This result is discussed in conjunction with other evidence that ATP may be a neurotransmitter substance released into the skin by impulses travelling antidromically in sensory axons.